The correct answer is GHz, which stands for gigahertz. It is the unit of frequency equivalent to one billion cycles per second and is commonly used to express the clock speed of CPUs. It indicates how many cycles a CPU can perform in a second, which relates to its overall performance. KB is a unit of storage capacity, not speed. Gbps measures data transfer rates for networks, not CPU speed. RPM is used for rotational speeds, such as in motors and hard disk drives, not for CPU clock speeds.